In an unprecedented campaign we ran tests on 106 brands to determine how strong they were, discovered which were best value and slammed rip-off brews.

We even set up a panel of dockers and lorry drivers to taste bitters in controlled conditions,

The story of how we raised the alarm and led a campaign to save real ale is among those told in a new book Brew Britannia.

After the Second World War small local breweries – fighting rising costs and falling demand – began to merge or be taken over.

By the 1960s a handful of big companies such as Whitbread and Watney’s – known as the Big Six – controlled almost the entire market and most of the pubs, too.

Local pubs were given corporate makeovers and the beer they sold was replaced by national brands backed by huge ad campaigns.

Men wept when they discovered the pint they had enjoyed for decades had gone.

Daily Mirror on 10th July 1972

And if you lived in a Watney’s town and didn’t like Watney’s, moving house was the only option. Looking to maximise profits, the conglomerates had declared war on what we now call “real ale”.

Unpasteurised and unfiltered, it relied on landlords’ expertise to ensure it was served in good condition. At its best it tasted sublime.

But at its worst – and it was, unfortunately, often badly looked after – it was a liability.

So enter the keg beer. It was the same basic brew only pasteurised, filtered, put into sealed metal containers and given its fizz from a tank of carbon dioxide instead of by the gentle activity of live yeast.

It was much more consistent and had a longer shelf life. Some of it didn’t even taste that bad but it was hardly likely to get anyone excited – a bit like processed cheese.

Drinkers brought up on characterful, lip-smacking bitter such as Young’s or Boddington’s hated it.

In the early 1970s, if you wanted to know the strength of a brew it was tough luck because manufacturers weren’t obliged to tell you.

It was at this point the Mirror turned its attention to beer with a series of in-depth articles spread across three days in July 1972.

The paper’s Square Deal Team, working with the Consumers’ Association, got hold of samples of 106 beers, from mild to lager, and had them laboratory tested. For each they established the original gravity – an indication of how much sugar went into the mix before fermentation – and percentage of alcohol in the final brew.

Those two figures give an idea of body and strength.

Daily Mirror on 10th July 1972

What they discovered was that, yes, beers had been getting gradually weaker over the preceding decade. They also found that the best, in terms of strength and value, were real ales from small independent breweries.

They declared Blue Anchor Spingo from Helston in Cornwall the best of the bitters.

It had plenty of body, 4.2% alcohol and a price of 2s 5d (12p) a pint. The strongest mild by a long chalk was from the tiny Batham’s brewery of Staffordshire. Keg beers were, on the whole, declared a rip off.

The Mirror wrote: “If the results of our test are anything to go by, then it is hard to understand why this kind of draught beer is becoming so popular.”

Alongside the test results, we put forward powerful arguments against the decline in choice and quality.

“The ardent beer drinker,” one piece said, “once had a choice of about 4,000 brews and now has only 1,500.”

We also explained the importance of traditional beer in British working-class culture and painted a picture of the long heritage represented in breweries such as Batham’s and their “simple brews drunk in Grandad’s day”.

It was stirring stuff.

Finally, just for a bit of fun, the paper got together a panel of six working men, including two dockers and a lorry driver, and got them to taste the bitters in controlled conditions over the course of a day.

The men found all the sniffing and sipping a bit silly – “It’s a funny way to drink beer!” But they finally concluded that the best brews by far were from small local firms, with Theakston of Masham, North Yorks, and Batham’s coming out on top.

At a time when small breweries were struggling to survive, this kind of support gave them the boost they desperately needed.

Daily Mirror on 11th July 1972

The Campaign for Real Ale had been founded in 1971 by four twenty-something friends from the North West on holiday in Ireland, initially as a bit of a joke.

In July 1972 it still had only around a hundred members and was considered by some to be pretty shambolic.

The Mirror’s high-profile investigation into the state of British beer played a part in Camra’s startling growth – it had a thousand members by the end of 1973 – and after that the real ale revolution was seriously under way.

Today Camra has more than 160,000 members and a record 158 new breweries have opened in the past year, the highest ever recorded in the Good Beer Guide.

Camra said it was an “astonishing” number, with twice as many now in operation than a decade ago – one for every 50 pubs.

The great British pint – as quintessential as fish and chips, missed penalties and moaning about the weather – is certainly in rude health again . We now sup more than 600 million of them every year.

So if you’re downing one in the pub tonight, don’t forget to raise your glass to the Mirror and say thanks for helping to save it.

Brew Britannia by Jessica Boak and Ray Bailey. Published by Aurum Press on June 19, £12.99.